The main structural support of the human skeleton is the spine which is the bone structure that extends from the base of the skull to the pelvis. It includes a spinal cord which is approximately eighteen inches in length and comprised of nerves that carry impulses to and from the brain to the rest of the body.
Surrounding the spinal cord are pairs of rings of bone called vertebrae which constitute the spinal column (back bones) and each pair of vertebrae is connected by a flexible joint that stabilizes the vertebrae and allows the spine to move.
An “intervertebral disk”—or simply the “disk”—is located between each pair of vertebrae within the flexible joint and bears most of the compressive load of the spinal column. Each disk is a flat, circular capsule approximately one inch in diameter and has an outer layer or membrane which is strong and flexible and comprised of a fibrous cartilage called the annulus fibrosis. It also has an inner core which consists of a soft, gelatinous substance called the nucleus pulposus. The main function of the disk is to cushion the vertebrae during movement.
The structure of the human spine is designed for an upright position, a typical posture for humans throughout history, where walking, running, hunting, gathering, working on farms or at workbenches were common body motions and positions. Today, a high proportion of people lead sedentary lives, spending the better part of each day sitting behind desks writing patent applications, at work stations, in automobiles, etc. These changes in human behavior overtime, mainly resulting from technological advances, have had a profound and largely negative impact on human physiology, and particularly the spine. As a result, spine or back problems are the most common physical complaints among adults.
Everyday physical stresses and the normal aging process also adversely affect the human spine. In that connection, one of the most common back problems experienced by adults results from degenerative disk disease, a general term applied to degeneration of the intervertebral disks. As the body ages, the disk material loses its elasticity and hardens, developing a consistency similar to a piece of hard rubber.
A specific example of degenerative disk disease is a herniated disk which is a condition resulting from strain or injury to the disk that causes the inner material of the disk to swell or herniate and the outer layer to rupture. When the disk ruptures, the inner material bulges and presses against, or pinches, the spinal nerves, resulting in severe pain.
When the disk degenerates to the point where it no longer properly functions, the disk is removed during a procedure called a diskectomy. A diskectomy involves removal of the ruptured or diseased disk from its location between adjacent vertebrae. By removing the disk and any associated disk or bone fragments, the source of the pressure on the spinal nerve is also removed, thereby relieving the pain.
Following a diskectomy, the adjacent vertebrae may be fused together, resulting in partial loss of spinal flexibility. On the other hand, a bone graft or other specialized material, such as a prosthetic intervertebral implant, may be placed in the empty disk space in order to stabilize the vertebrae.
Bone grafts and similar prosthetic implants used following diskectomy require the implant and surrounding vertebrae to be shaped using precision drilling and shaving techniques in order to provide a proper fit with the implant. This type of surgical reconstruction is difficult and time-consuming and often still results in limited flexibilty of the spine. As a result, synthetic intervertebral disk prostheses have been developed such as those described in U.S. Pat. No. 4,863,477. These synthetic prostheses are fabricated prior to performance of the surgery and are shaped during surgery to conform specifically to the shape of the disk space, thereby eliminating the tedious task of precision drilling and shaving techniques associated with bone implants. Moreover, these synthetic prostheses provide a resiliency that facilitates flexibility of the spine.
In order to ensure that the prosthetic incorporates the proper shape and volume for the target space, various measuring techniques have been proposed. These techniques include X-rays, magnetic resonance imaging (MRI), computed tomography (CT) scans and myelography, a radiological technique for viewing the spinal cord. These techniques, although quite useful, are not without certain drawbacks including high costs, potential adverse side effects and inherent measuring inaccuracies which result from a variety of factors, including high signal to noise ratios, limited two-dimensional images, and potential radiation exposure. Furthermore, these devices are expensive and require highly-skilled technicians to operate them properly.
As a result, practitioners and medical institutions have continually sought a lower cost and less complex method of obtaining the data necessary to fabricate a quality prosthetic. In particular, there is a desire to obtain low-cost vet highly accurate body cavity measuring device that can be used with minimal to no side effects. Such a device must be biocompatible, non-toxic and simple to use. Finally, such a device must be fabricated by a manufacturing method that is efficient, easy to implement and cost effective.